1. Field of the Invention
The Invention relates to optical modulators and, more particularly, to optical modulators having optimized electro-optical interaction to permit reduced power requirements.
2. Description of the Related Art
Traditional cable TV systems (often referred to as "CATV systems") used coaxial cable to provide up to about eighty channels of broadcast video signals. Recently, however, consumers are demanding more channels and additional services, such as high-definition television. In order to provide the increased signal capacity required by these demands, CATV systems have begun to employ fiber optical cables which carry signals as beams of light.
One drawback of optical fiber transmission systems, both analog and digital, is that the fiber optical cables are subject to a phenomenon known as "Stimulated Brillouin Scattering" (SBS). This is a nonlinear effect that occurs when the optical power at the fiber entrance exceeds a given threshold (typically about 6 dBm for a narrow bandwidth source having a spectral bandwidth of less than 20 MHz), thus producing an intense optical field which generates an acoustic wave in the fiber. The acoustic wave acts as a dynamic diffraction grating, generating a reflected wave that takes power away from the optical signal field. This phenomenon is discussed in U.S. Pat. No. 4,560,246.
One technique for suppressing Brillouin scattering, both in analog and digital systems, consists in phase modulating the optical signal field. The phase modulation has no influence on the detection process at the receiver, but it induces a broadening of the spectrum of the signal transmitted in the fiber beyond the coherence band of Brillouin scattering.
For the phase modulation to produce a good suppression of Brillouin scattering, the modulation frequency must be sufficiently high and the phase deviation be above 2.pi..
The power necessary to obtain such a phase modulation is generally high (greater than 5W at a phase modulation frequency of 2 GHz). This limits the applications in which the phase modulation technique can be used. For example, an increase in the power leads to heat generation inside the electrodes fabricated on the lithium niobate substrate typically used in designing the modulators. This material has a low thermal conductivity, and thus heat generated in the material tends to build up, causing an increase in the operating temperature and thus reducing reliability.
An optical phase modulator of lithium niobate (LiNbO.sub.3) is typically obtained by utilizing the electro-optical effect of the material. This effect consists in modifying, through an applied electric field, the index of refraction of the optical guide in which the optical signal is propagated. The variation in time of the refractive index produces the desired phase modulation of the optical field.
To obtain a phase modulator, it is necessary to have an optical waveguide and an electrode structure that permits the generation of the electric field necessary for the modulation. Optical waveguides in the LiNbO.sub.3 crystal are generally formed by titanium diffusion or by proton exchange.
Lumped and traveling wave electrode structures are known. Examples of the two types of electrode structures for optical modulators 10 and 10a are shown in FIGS. 1A and 2A. Each modulator includes an optical waveguide 12 formed in a substrate 14. A pair of electrodes 16, 18 are formed on the surface of substrate 14 on opposite sides of waveguide. The two configurations differ essentially in the electrical termination. The basic characteristics and operating parameters of such configurations are well known in the prior art. A general reference is, for example, chapter 4 by R. C. Alferness "Titanium Diffused Lithium Niobate Waveguide Devices", in a book entitled "Guided-Wave Optoelectronics" (Ed. T. Tamir) published in 1988, Springer Verlag.
Lumped electrodes are shown in FIG. 1A. Electrode 16 is connected to a source, or driver, of a modulating signal V and electrode 18 is connected to ground. A lumped electrode type of modulator functions best at low frequencies. In fact, from the electrical standpoint, the lumped electrode structure behaves essentially as a concentrated capacitor C that, together with the internal resistance R of the driver and of the electrodes, constitutes a low-pass RC filter, as shown in FIG. 1B.
The traveling-wave type of electrode structure overcomes this limitation. Electrodes 16 and 18 of this structure, shown in FIG. 2A, constitute a transmission line terminated in its characteristic impedance R.sub.c, that is, a matched line. The equivalent electric circuit is shown in FIG. 2B. The electrooptical response of the traveling-wave type of modulator is also of the low-pass type, but the cut-off frequency is determined by the difference in velocity between the wave of the optical signal and the wave of the modulating electric field. This velocity difference can be made quite small and thus these modulators generally have a higher cutoff frequency.
Both types of modulators have an electrooptical response of the low-pass type.
U.S. Pat. No. 4,372,643 (Liu et al.) discloses an ultrafast gate produced by locally modulating the coupling along a pair of coupled wavepaths by means of a standing-wave electrical signal. The electrodes form an electrical transmission line that is energized at its input by means of a signal source having an output impedance R. In one embodiment, the transmission line is terminated by means of a short circuit and the electrodes are proportioned such that the input impedance of the line has a real part that is equal to R. When energized, a standing wave is produced along the length of the electrodes which locally affects the coupling between the optical waveguides. Alternatively, the electrodes can be terminated by means of an open circuit.
U.S. Pat. No. 4,850,667 (Djupsjobacka) relates to an electrode arrangement for optoelectronic devices. A first elongate electrode has a connecting conductor for an incoming microwave signal with the aid of which a light wave is to be modulated. The connecting conductor divides the first electrode into a standing wave guide and a traveling wave guide, which is connected via a resistor to a U-shaped second electrode. It is stated in the patent that the incoming modulating microwave has maximum modulating ability in the standing waveguide if its frequency is in agreement with the resonance frequency f.sub.c of the standing wave guide.
In an embodiment, the standing wave guide is connected at one end with the U-shaped second electrode at its closed end. The standing waveguide has in this embodiment a resonance frequency f.sub.c =c/(4.multidot.L.multidot.n.mu.), where c is the speed of light in vacuum, L is the length of the standing wave guide and n.mu. is the refractive index for the microwave guides.
SU patent 696842 discloses electro-optical ultra-high-frequency light modulators based on a bulk electro-optical crystal. Two coaxial resonators are coupled to the ends of two electrodes applied on opposite faces of the electro-optical crystal. Modulator efficiency and modulation depth are increased by short-circuiting opposite ends of the electrodes. The electrical length of the coaxial resonators and of the electrodes is respectively equal to one quarter wavelength and half wavelength of the center frequency of the modulator's working range. The coaxial resonators can be replaced by microstrip resonators.
A paper by G. K. Gopalakrishnan et al., IEEE Transactions on Microwave Theory and Techniques, Vol. 42, No. 12, Dec. 1994, pag. 2650-2656, discloses the performance and modeling of resonantly enhanced LiNbO.sub.3 traveling wave optical modulators. A resonant enhancement technique involving external line stretching of a length of the nonactive section of the modulator is proposed and demonstrated at low frequencies.
A paper by M. M. Hoverton et al., Journal of Lightwave Technology, vol. 14. no. 3, March 1996, pag. 417-422, discloses SBS suppression using a depolarized source for high power fiber applications. A depolarized source is demonstrated using a traveling wave LiNbO.sub.3 phase modulator. Resonant enhancement of the low frequency response of the modulator is obtained by using a non resonant traveling wave modulator along with a length of coaxial cable that is appropriately terminated in an open or short circuit. Thus the composite resonant cavity in this arrangement comprises the active section of the modulator and the nonactive coaxial cable.
Applicants have observed that conventional structures of the low-pass type for providing electro optical modulators do not meet the needs of current applications for optical phase modulators.
Applicants have recognized that less than optimum modulation performances are achieved in band-pass type electro-optical modulators if only the electrical resonance of the modulating signal in the electrode structure, or the power coupling between the electrical generator and the electrode structure are maximized (electrical efficiency), without regard for the electrooptical interaction.
Applicants have discovered that a more efficient modulation can be achieved by optimizing the electrooptical interaction of the modulating electrical signal with the light beam propagating in the optical waveguide structure.
Applicants have further discovered apparatus for providing an optical modulator with an electrooptical response of the band-pass type which optimizes the electrooptical interaction at a central frequency. This apparatus makes it possible to obtain the same depth of modulation at the working frequency with a lower power.